Field of the Invention
This invention generally relates to woven preforms and particularly relates to woven preforms used in reinforced composite materials, which can be woven flat and folded into its final shape.
Incorporation by Reference
All patents, patent applications, documents, references, manufacturer's instructions, descriptions, product specifications, and product sheets for any products mentioned herein are incorporated herein by reference, and may be employed in the practice of the invention.
Description of the Prior Art
The use of reinforced composite materials to produce structural components is now widespread, particularly in applications where their desirable characteristics are sought of being light in weight, strong, tough, thermally resistant, self-supporting and adaptable to being formed and shaped. Such components are used, for example, in aeronautical, aerospace, satellite, recreational (as in racing boats and autos), and other applications.
Typically such components consist of reinforcement materials embedded in matrix materials. The reinforcement component may be made from materials such as glass, carbon, ceramic, aramid, polyethylene, and/or other materials which exhibit desired physical, thermal, chemical and/or other properties, chief among which is great strength against stress failure. Through the use of such reinforcement materials, which ultimately become a constituent element of the completed component, the desired characteristics of the reinforcement materials, such as very high strength, are imparted to the completed composite component. The constituent reinforcement materials typically, may be woven, knitted or otherwise oriented into desired configurations and shapes for reinforcement preforms. Usually particular attention is paid to ensure the optimum utilization of the properties for which the constituent reinforcing materials have been selected. Usually such reinforcement preforms are combined with matrix material to form desired finished components or to produce working stock for the ultimate production of finished components.
After the desired reinforcement preform has been constructed, the preform may be impregnated in the matrix material, so that typically the reinforcement preform becomes encased in the matrix material and matrix material fills the interstitial areas between the constituent elements of the reinforcement preform. The matrix material may be any of a wide variety of materials, such as epoxy, polyester, vinyl-ester, ceramic, carbon and/or other materials, which also exhibit desired physical, thermal, chemical, and/or other properties. The materials chosen for use as the matrix may or may not be the same as that of the reinforcement preform and may or may not have comparable physical, chemical, thermal or other properties. Typically, however, they will not be of the same materials or have comparable physical, chemical thermal or other properties, since a usual objective sought in using composites in the first place is to achieve a combination of characteristics in the finished product that is not attainable through the use of one constituent material alone. So combined, the reinforcement preform and the matrix material may then be cured and stabilized in the same operation by thermosetting or other known methods, and then subjected to other operations toward producing the desired component. It is significant to note at this point that after being so cured, the then solidified masses of the matrix material normally are very strongly adhered to the reinforcing material (e.g., the reinforcement preform). As a result, stress on the finished component, particularly via its matrix material acting as an adhesive between fibers, may be effectively transferred to and borne by the constituent material of the reinforcement preform.
Frequently, it is desired to produce components in configurations that are other than such simple geometric shapes as (per se) plates, sheets, rectangular or square solids, etc. A way to do this is to combine such basic geometric shapes into the desired more complex forms. One such typical combination is made by joining reinforcement preforms made as described above at an angle (typically a right-angle) with respect to each other. Usual purposes for such angular arrangements of joined reinforcement preforms are to create a desired shape to form a reinforcement preform that includes one or more end walls or “T” intersections for example, or to strengthen the resulting combination of reinforcement preforms and the composite structure which it produces against deflection or failure upon it being exposed to exterior forces, such as pressure or tension. In any case, a related consideration is to make each juncture between the constituent components as strong as possible. Given the desired very high strength of the reinforcement preform constituents per se, weakness of the juncture becomes, effectively, a “weak link” in a structural “chain.”
An example of an intersecting configuration is set forth in U.S. Pat. No. 6,103,337, the disclosure of which is incorporated herein by reference. This reference sets forth an effective means of joining together two reinforcing plates into a T-form.
Various other proposals have been made in the past for making such junctures. It has been proposed to form and cure a panel element and an angled stiffening element separate from each other, with the latter having a single panel contact surface or being bifurcated at one end to form two divergent, co-planar panel contact surfaces. The two components are then joined by adhesively bonding the panel contact surface(s) of the stiffening element to a contact surface of the other component using thermosetting adhesive or other adhesive material. However, when tension is applied to the cured panel or the skin of the composite structure, loads at unacceptably low values resulted in “peel” forces which separate the stiffening element from the panel at their interface since the effective strength of the joint is that of the adhesive and not of the matrix material.
The use of metal bolts or rivets at the interface of such components is unacceptable because such additions at least partially destroy and weaken the integrity of composite structures themselves, add weight, and introduce differences in the coefficient of thermal expansion as between such elements and the surrounding material.
Other approaches to solving this problem have been based on the concept of introducing high strength fibers across the joint area through the use of such methods as stitching one of the components to the other and relying upon the stitching thread to introduce such strengthening fibers into and across the juncture site. One such approach is shown in U.S. Pat. No. 4,331,495 and its divisional counterpart, U.S. Pat. No. 4,256,790. These patents disclose junctures having been made between a first and second composite panel made from adhesively bonded fiber plies. The first panel is bifurcated at one end to form two divergent, co-planar panel contact surfaces in the prior art manner, that have been joined to the second panel by stitches of uncured flexible composite thread through both panels. The panels and thread have then been “co-cured,” i.e. cured simultaneously. Another method to improve upon junction strength is set forth in U.S. Pat. No. 5,429,853.
While the prior art has sought to improve upon the structural integrity of the reinforced composite and has achieved success, particularly in the case of U.S. Pat. No. 6,103,337, there exists a desire to improve thereon or address the problem through an approach different from the use of adhesives or mechanical coupling. In this regard, one approach might be by creating a woven three dimensional (“3D”) structure by specialized machines. However, the expense involved is considerable and rarely is it desirable to have a weaving machine directed to creating a simple structure. Despite this fact, 3D preforms which can be processed into fiber reinforced composite components are desirable because they provide increased strength relative to conventional two dimensional laminated composites. These preforms are particularly useful in applications that require the composite to carry out-of-plane loads. However, the prior-art preforms discussed above have been limited in their ability to withstand high out-of-plane loads, to be woven in an automated loom process, and to provide for varying thickness of portions of the preform.
Another approach would be to weave a two dimensional (“2D”) structure and to fold it into 3D shape. Fiber preforms with specific structural shapes, such as for example ‘T’, ‘I’, ‘H’ or ‘Pi’ cross sections, can be woven on a conventional shuttle loom, and several existing patents describe the method of weaving such structures (U.S. Pat. Nos. 6,446,675, 6,712,099, and 6,874,543, for example).
Many composite structures utilize Pi shaped preforms to join two or more perpendicular composite panels. A cross-sectional view of a typical Pi-shaped preform 100 is shown in FIG. 1, for example. The preform typically has two upstanding legs 125, 135 in the vertical position, forming a clevis 116 between legs 125, 135. However, the legs 125, 135 may be perpendicular or non-perpendicular or angled to base 120. Preform 100 is woven by repeating the complete weave sequence to form adjacent vertical sections along the longitudinal length of preform 100. The weave process produces continuous lengths of preform 100, which are then cut to the desired lengths for installation.
In aircraft construction, the use of Pi preforms has proven to be an effective way to transfer loads from one component, for example, the web of the frame or spar, into another component, for example, a fuselage or wing skin. This is especially true for structures that subject the Pi to axial tension/compression, pull-off, and/or side bending loads.
In many applications, the Pi preform has to transfer shear loads between the two components, such as that shown in FIG. 2, for example. FIG. 2 depicts a T-shear loading configuration, wherein Pi preform 100 is used to join perpendicular panels 30, 40, for example, and the shear load 50 is being applied in an opposite direction indicated using bold arrows. In such a configuration, the traditional Pi, however, is not nearly as efficient for this type of loading as it is for the other load cases. The traditional Pi preform has reinforcement along the length of the preform (axially) and in the transverse direction (along the width of the base and along the height of the upstanding legs). This type of fiber orientation in a structure is commonly referred to as 0, +/−90 degree orientation. This type of orientation provides fiber that is parallel to the principal loading directions for the axial, pull off, and side bending cases. However, it does not provide any fiber in the off axis directions that are important for carrying shear loads as described in FIG. 2 above.